1. Field of the Invention
The present invention relates to the electrochemical production of an aqueous hydrogen peroxide solution and the composition of gas diffusion electrodes used for generating high yields of hydrogen peroxide.
2. Background of the Related Art
Hydrogen peroxide has strong oxidizing properties and does not leave a chemical residue. Accordingly, hydrogen peroxide has been found to be useful in many applications, such as the bleaching of paper, disinfection of wounds and medical devices, water treatment, decontamination of pathogenic agents, destruction of environmental wastes, and other applications. The various applications for hydrogen peroxide have their own unique requirements, but it is often beneficial to produce hydrogen peroxide on demand or at the point of use to avoid logisitical, cost, and safety issues associated with shipping and avoid the need to add stabilizing agents to the hydrogen peroxide solution which limit degradation of hydrogen peroxide.
For these and other reasons, electrochemical methods and apparatus for the synthesis of hydrogen peroxide have been developed. Many of these electrochemical methods are designed to convert water to oxygen and protons at the anode and convert oxygen and protons to hydrogen peroxide at the cathode when electrical current or potential is applied between the anode and cathode of a suitable electrochemical cell. Electrochemical generation of hydrogen peroxide has been performed in both acidic and alkaline solutions, as described in Tatapudi and Fenton, J. Electrochem. Soc. 140, L55-L57, 1993; Gupta and Oloman, J. Appl. Electrochem. 36, 255-264, 2006; Brillas, et. al. Electrochem. Acta 48, 331-340, 2002; and Gyenge and Oloman, J. Appl. Electrochem. 33, 665-663 (2003). Rather than using a “flow-through” electrochemical reactor which flows a liquid acidic or basic aqueous electrolyte to the cathode, there are specific advantages to generating hydrogen peroxide using a gas diffusion electrode which flows gas to the cathode. A gas diffusion electrode used in combination with a polymer electrolyte membrane allows the hydrogen peroxide to be generated and collected without additional acid or base present, which can be desirable for many applications since the acid and base can corrode or damage system components that use the produced hydrogen peroxide, such as a hydrogen peroxide vaporization system for decontamination and other uses. Another advantage of generating hydrogen peroxide using a gas diffusion electrode is that the rate of hydrogen peroxide production for a given cell area can be higher than when using a “flow-through” electrochemical reactor based on the significantly higher diffusion coefficient of oxygen in the gas phase (˜10−5 m2/s) compared to the aqueous phase (˜10−10 m2/sec). The mass transport limitations caused by water “flooding” within hydrogen-oxygen fuel cell cathodes has been well documented, as described in Baschuk and Li, J. Power Sources 86, 181-196, 2000. In addition, within a gas diffusion electrode, high concentrations of hydrogen peroxide can be generated continuously without requiring recirculation and then collection of an electrolyte solution in a “batch mode” configuration.
U.S. Pat. No. 5,972,196 (Murphy, et al.) describes that hydrogen peroxide can be generated electrochemically using a gas diffusion cathode and an anode separated by a cation-exchange polymer electrolyte membrane. This design allows hydrogen peroxide to be generated at the cathode within a gas diffusion electrode without the presence of a liquid electrolyte. For the operation of the electrochemical cell, protons are generated at the anode using either water (Eq. 1) or hydrogen (Eq. 2)Anode: 2H2O→O2+4H+4e− E°=−1.229 V (25° C.)  (Eq. 1)Anode: 2H2→4H+4e− E°=0.000 V(25° C.)  (Eq. 2)Protons from the anode are transferred across the cation-exchange membrane to the cathode compartment towards the negatively charged electrode. The cathode compartment is fed with oxygen or air for the generation of hydrogen peroxide by the reduction of oxygen according to the following reaction.Cathode: O2+2H+2e−→H2O2 E°=0.682 V (25° C.)  (Eq. 3)The hydrogen peroxide reaction product must be promptly removed from the vicinity of the cathode to prevent further reduction. An alternative side reaction (Eq. 4), listed below, produces water rather than hydrogen peroxide.Cathode: O2+4H+4e−→2H2O E°=1.229 V (25° C.)  (Eq. 4)In addition, further reduction of hydrogen peroxide may also occur, as described by the following equation (Eq. 5).Cathode: H2O2+2H+2e−→2H2O E°=1.776 V (25° C.)  (Eq. 5)The specific construction and composition of the cathodic electrode must be optimized to reduce decomposition of hydrogen peroxide through electroreduction via potential side reactions (Eq. 4 and Eq. 5). In addition, the cell design and components must be optimized to minimize hydrogen peroxide decomposition via non-Faradic processes such as surface-catalyzed decomposition at endplates and other cell components. The design of the components of an electrochemical cell is critically important to the hydrogen peroxide concentration, current efficiency, and long term operation of the cell. In particular, since hydrogen peroxide is synthesized at the cathode of an electrochemical cell, there has been significant research and development directed at cathode designs for increased production of hydrogen peroxide.
U.S. Pat. No. 5,972,196 (Murphy, et al.) discloses an electrochemical cell for the generation of ozone at the anode and the generation of either water or hydrogen peroxide at the cathode. The electrochemical cell has a gas diffusion cathode electrode comprising a semi-hydrophobic catalyst layer supported on a hydrophobic gas diffusion layer of carbon cloth or carbon fiber paper. The hydrophobic gas diffusion layer has a carbon cloth or carbon fiber paper impregnated with a sintered mass derived from fine carbon powder and a polytetrafluoroethylene emulsion. The semi-hydrophobic catalyst layer may comprise a proton exchange polymer, polytetrafluoroethylene and a high surface area carbon-supported, pyrolyzed cobalt porphyrin, such as cobalt tetrakis(4-methoxyphenyl) porphyrin (CoTTMP). This electrochemical cell was shown to produce hydrogen peroxide concentrations up to about 1.4 wt. %.
U.S. Pat. No. 6,555,055 (Cisar, et al.) discloses an electrochemical cell for the electrochemical production of hydrogen peroxide. The electrolyzer includes a cathode catalyst composed of cobalt (II) tetrakis-(4-methoxyphenyl)-porphine (CoTMPP) which was adsorbed onto high surface area carbon black and then pyrolyzed. The catalyst was suspended in a Nafion/water mixture before painting and hot pressing onto the membrane. An aqueous hydrogen peroxide solution was produced having a hydrogen peroxide concentration as high as 2.2 wt. %.
U.S. Pat. No. 6,712,949 (Gopal) discloses a cathode structure for use in electrochemical synthesis of hydrogen peroxide. A redox catalyst is mixed with carbon, PTFE, and a performance modifier or enhancer such as a quaternary ammonium compound. This mixture is then directly deposited on a high surface area carbon felt or porous carbon cloth. The resulting cathode may be used in combination with an ion exchange membrane and an anode for oxidization of water to produce oxygen and protons. In contrast to the previous two examples cited, the cathode is utilized in an electrochemical cell with anolyte and catholyte solutions circulating through anolyte and catholyte compartments separated by a proton exchange membrane. Hydrogen peroxide concentrations as high as about 7 wt. % are reported to have been achieved in an acidic solution (1 NH2SO4) that contained dissolved oxygen in solution. Within this configuration, the components used were not designed for use within a gas diffusion electrode where flowing gas (air or oxygen) is used within the cathode rather than a flowing aqueous solution. The presence of a flowing acidic solution within the cathode influences both the generation of hydrogen peroxide and its removal from the electrode. The acidic solution has a high concentration of mobile protons and can also contribute to preventing the hydrogen peroxide from decomposing as described above.
U.S. Pat. No. 6,712,949 (Gopal) also discloses the use of high molecular weight organic compounds and polymers including poly(2-vinylpyridine) poly(4-vinylpyridine), poly(4-vinylpyridinium tribromide), poly(4-vinylpyridine) methyl chloride quaternary salt, and poly(4-vinylpyridinium p-toluenesulfonate) as “performance modifiers” within cathodes used for the electrochemical production of hydrogen peroxide. The concentration of hydrogen peroxide produced was higher for electrodes containing the “performance modifiers” compared to electrodes without this component.
The use of surfactants or “additives” including trioctylmethylammonium chloride has been shown to influence the hydrogen peroxide concentration and current efficiency within “flow-through” electrochemical reactors (Gyenge and Oloman, J. Electrochem. Soc. 152, D42-D53, 2005). Similar to U.S. Pat. No. 6,712,949, the flow-through reactor process involved the use of flowing acidic or basic solution within the cathode.
There remains a need for a cathodic gas diffusion electrode and specific components within the electrode which allow high concentrations of hydrogen peroxide to be produced and allow removal of the hydrogen peroxide to prevent its decomposition. It would be desirable for the electrode, method and apparatus to produce high concentrations of hydrogen peroxide at high current efficiencies over an extended period of operation.